1. Field of the Invention
The present invention relates to a multi-function machine tool provided with a single spindle head having a rotating spindle to which a plurality of different sub-motor units are changeably attached in response to a change in the machining operations to be applied to a workpiece, the machining conditions, and the kind of material of the workpiece. Each sub-motor unit accommodates therein a tool for an intended machining operation such as milling operation, boring operation or grinding operation, and a drive motor for driving the tool at a predetermined rotating speed and a predetermined output torque.
2. Description of the Related Art
It is well known that a machine tool, particularly a milling machine, is provided with a spindle head having a spindle which is rotated at a selected speed to effect diverse kinds of machining operations, such as a milling operation, a grinding operation and a boring operation, by the use of tools changeably attached to the end of the spindle. The spindle head is provided with a drive motor mounted on the top thereof or built in the housing thereof for driving the rotation of the spindle together with the tool attached thereto.
A machine tool with an automatic tool changer has been also known and used. The automatic tool changer, having a tool magazine storing therein a plurality of diverse tools, can automatically change a tool attached to the end of a spindle of the machine tool to a different tool selected from the plurality of tool stored in the magazine. Thus, the machine tool with the automatic tool changer can perform a plurality of different machining operations often automatically changing a tool, attached to the spindle, to another tool selected from the tools stored in the magazine of the automatic tool changer.
Further, a more fully automated machine tool, referred to as a machining center, is well known and is a completely automated machine tool which is capable of conducting a plurality of machining operations according to a stored program and by using one of a plurality of tools changed by an automatic tool changer.
In the described known machine tools, a drive motor, i.e., a spindle motor for driving the rotation of the spindle, consists of a servo motor which exhibits a predetermined rated speed and a predetermined rated output power designed in accordance with the expected use of the machine tool. Nevertheless, either when the kind of material of the workpiece to be machined by the machine tool greatly changes or when the operating condition of the machine tool in which a workpiece is machined greatly vary, the single spindle motor rotationally driving the spindle cannot adequately operate so as to comply with large changes and variations. Namely, when a workpiece made of a non-free-cutting metallic material such as iron-system metal is machined by a machine tool, the spindle motor must be able to exhibit a large output torque at a relatively low rotating speed. On the other hand, when a workplace made of a soft free-cutting material such as aluminum or plastic is machined, the spindle motor must be able to rotate the spindle at a very high speed. Thus, the operating conditions required for the spindle motor are different from one another and, accordingly, it is usually impossible for one single spindle motor to appropriately operate at many different operating conditions.
Further, during machining operations by the machine tool, it is often required that the same workplace receives both rough machining and finish-machining by the same machine tool. However, the single drive motor of the machine tool cannot drive the spindle at both rough machining and finish-machining speeds and different output torques.
From the viewpoint of increasing machining efficiency of a machine tool, a tool is required to be driven, in some cases, by a drive motor having a high acceleration performance and, in other cases, by a different drive motor having a high deceleration performance. Obviously, one single drive motor mounted on the spindle head of a machine tool is unable to comply with such requirements. Nevertheless, the conventional machine tool is not provided with any means for supplement the performance of the spindle motor.